Hey there! I'm happy to help!
In our problem, 100 is a certain percent of 80. When talking about percents, the word "is" means equals. Let's represent our certain percent with p and solve for it.
100=80p
Let's flip the equation around so p is on the left side.
80p=100
We divide both sides by 80.
p=1.25
As a percent, the increase is 125%.
Have a wonderful day!